Inamethacholinechallengetest,spirometrymeasurementsaremadeafterthepatientinhalesincreasingconcentrationsofthemuscarinicagonistmethacholine.ThetestisstoppedwhenFEV1fallsby20%ormoreorwhenamaximumconcentration(25mg/mL)ofmethacholinehasbeeninhaled.Theconcentrationofmethacholinethatproducesa20%decreaseinFEV1iscalledtheprovocation concentration (PC)20. ThelowerthePC20,themoresensitiveanindividualistomethacholine.MostindividualswithasthmahaveaPC20lessthan8mg/mLofmethacholine.
Determinants of Maximal Flow
The shape of the flow-volume loop reveals important information about normal lung physiology that can be altered by disease. Inspection of the flow-volume loop reveals that the maximum inspiratory flow is the same or slightly greater than the maximum expiratory flow. Three factors are responsible for the maximum inspiratory flow. First, the force generated by the inspiratory muscles decreases as lung volume increases above RV. Second, the recoil pressure of the lung increases as the lung volume increases above RV. This opposes the force generated by the inspiratory muscles and reduces maximum inspiratory flow. However, airway resistance decreases with increasing lung volume as the airway caliber increases. The combination of inspiratory muscle force, recoil of the lung, and changes in airway resistance causes maximal inspiratory flow to occur about halfway between TLC and RV.
During exhalation, maximal flow occurs early (in the first 20%) in the maneuver, and flow rates decrease progressively toward RV. Even with increasing effort, maximal flow decreases as RV is approached. This is known as expiratory flow limitation and can be demonstrated by asking an individual to perform three forced expiratory maneuvers with increasing effort.
Fig. 22.4 shows the results of these three maneuvers. As effort increases, peak expiratory flow increases. However, the flow rates at lower lung volumes converge; this indicates that with modest effort, maximal expiratory flow is achieved. No amount of effort will increase the flow rates as lung volume decreases. For this reason, expiratory flow rates at lower lung volumes are said to be effort independent and flow limited because maximal flow is achieved with modest effort, and no amount of additional effort can increase the flow rate beyond this limit. In contrast, events early in the expiratory maneuver are said to be effort dependent; that is, increasing effort generates increasing flow rates. In general the first 20% of the flow in the expiratory flow-volume loop is effort dependent.
Flow Limitation and the Equal Pressure Point
Why is expiratory flow limited and reasonably effort independent? Factors that limit expiratory flow are important because many lung diseases affect these factors and thus affect the volume and speed with which air is moved into and out of the lung. Flow limitation occurs when the airways, which are intrinsically floppy distensible tubes, become compressed. The airways become compressed when the pressure outside the airway exceeds the pressure inside the airway. How and when this occurs is important to understanding lung disease.
Fig. 22.5 shows the events that
TLC, no flow •Fig. 22.5 Flowlimitation.A,Endinspiration,beforethestartofexhalation.B,Atthestartofaforcedexhalation.C,Expiratoryflowlimitationlaterinaforcedexhalation.Expiratoryflowlimitationoccursatlocationswhereairwaydiameterisnarrowedasaresultofnegativetransmuralpressure.Seetextfordetails.